This application relates to coupling of optical energy from one optical device to another, and more particularly, to techniques and devices for coupling optical energy based on evanescent waves such as coupling optical energy into or out of a whispering-gallery-mode cavity.
Many optical devices and systems involve coupling optical energy from one optical element or device to another optical element or device. An optical device that accomplishes such optical coupling is an optical coupler. For example, a partially reflective and partially transmissive reflector, a beam splitter, or a direction fiber coupler may be used as an optical coupler in various applications.
Some optical couplers are designed to couple optical energy by evanescent coupling in which an optical interface is provided to allow optical energy in an evanescent field of an optical signal in one optical element or device to be extracted and received by another optical element or device. An optical prism, for example, may be used to operate as such an optical coupler when an optical signal received by the prism is totally internally reflected at one prism facet. The energy distribution of the totally reflected beam, however, is not entirely confined within the prism but penetrates through that prism facet to extend outside the prism in a near-field region on the order of or less than one wavelength of the optical signal. The optical field outside the prism facet is the evanescent field and decays exponentially with the distance from the prism facet. An optical element or device can be placed in the near-field region to receive a portion of the reflected optical signal.
One application of the optical evanescent coupling is whispering-gallery-mode optical cavities. Whispering-gallery-mode optical cavities formed from glass micro spheres have been demonstrated to exhibit high quality factors (Q's). See, e.g., Gorodetsky et al., “Ultimate Q of Optical Microsphere Resonators,” Optics Letters, Vol. 21(7), pp.453-455 (1996). The microsphere can be designed to support a special set of resonator modes known as “whispering gallery modes.” A whispering gallery mode is essentially an electromagnetic field mode that is confined in an interior region close to the surface of the sphere around its equator and circulates by total internal reflection at the sphere surface. Light need be launched into the cavity from a phase-matched evanescent wave in order to excite a whispering gallery mode. Likewise, light in a whispering gallery mode in the cavity need be coupled out of the cavity by evanescent coupling.
Such evanescent coupling may be achieved in a number of ways. One technique, for example, uses a prism spaced from the micro sphere by a sub-micron gap to produce an evanescent in the gap by total internal reflection in the prism. See, e.g., Gorodetsky et al., supra. and Sandoghdar et al., “Very Low Threshold Whispering-Gallery-Mode Microsphere Laser,” Physics Review A, Vol.54(3), pp. 777-1780 (1996). In another technique, a tapered single-mode fiber coupler is formed by pulling the fiber under a heated condition to form a narrow neck of the tapered portion (e.g., about 1 to 3 microns) to allow for evanescent leaking of the guided optical energy at the neck to couple energy into or out of the micro sphere. See, e.g., Knight et al., Optics Letters, Vol.22 pp.1129-1131 (1997) and Cai et al., “Fiber-Optic Add-Drop Device based on A Silica Microsphere-Whispering Gallery Mode System,” IEEE Photon. Technol. Lett., Vol.11(6), pp.686-687 (1999).